RFR: JDK-8257086: Clarify differences between {Float, Double}.equals and ==

[Joe Darcy]

Updates to the specifications of Double.{equals, compareTo} to explain more explicitly why the obvious wrappers around "==" and "<"/"==" are not sufficient for floating-point values.

Once the wording is worked out, I'll replicate it for the analogous methods on Float.

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Commit messages:
 - Fix whitespace
 - Initial work for JDK-8257086.

Changes: https://git.openjdk.java.net/jdk/pull/1699/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1699&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257086
  Stats: 65 lines in 1 file changed: 47 ins; 0 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1699.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1699/head:pull/1699

PR: https://git.openjdk.java.net/jdk/pull/1699